Integrated electrical circuits and other microstructured components are usually produced by virtue of a plurality of structured layers being applied onto a suitable substrate, which can be a silicon wafer, for example. For the purposes of structuring the layers, these are initially covered by a photoresist (resist), which is sensitive to light from a specific wavelength range, e.g. light in the deep ultraviolet (DUV), vacuum ultraviolet (VUV) or extreme ultraviolet (EUV) spectral range. Subsequently, the wafer thus coated is exposed in a projection exposure apparatus. Here, a mask with a pattern of structures is illuminated by an illumination system and imaged on the photoresist with the aid of a projection lens. Since the absolute value of the imaging scale generally is less than 1 in this case, such projection lenses are sometimes also referred to as reduction lenses.
The wafer is subjected to an etching process after developing the photoresist, as a result of which the layer is structured in accordance with the pattern on the mask. The photoresist which still remains is then removed from the remaining parts of the layer. This process is repeated until all layers are applied to the wafer.
In order to be able to image the pattern on the mask on to the photoresist in an ideal manner, the mask is illuminated, in general, with an illumination angle distribution that is specifically adapted to the pattern. The term illumination angle distribution is understood to mean the distribution of the directions of the light rays when they are incident on the mask.
A spatial distribution in a pupil plane of the illumination system, which has a Fourier relationship with the mask plane, corresponds to the illumination angle distribution in the mask plane. Therefore, the corresponding spatial distribution in the pupil plane is often resorted to for the purposes of describing the illumination angle distribution in the mask plane. In the case of an annular illumination setting, a ring-shaped region is illuminated in the pupil plane, for example. For the illumination angle distribution, this means that the projection light is only incident obliquely on the individual field points. Here, the occurring angles are set by the inner and outer radii of the ring that illuminates the pupil. In the case of a multi-pole illumination, there only is illumination of individual poles that are separated from one another in the pupil plane of the illumination system. In this case, the projection light assigned to a pole is incident on the mask at comparatively large angles, but these only differ a little from one another.
A difficulty in the lithographic production of microstructured components involves exactly aligning the structures of adjacent layers with respect to one another. Similar issues arise if the structures within a single layer are produced by multiple exposure. Then, the structures that were defined by a first exposure step have to be aligned very exactly with respect to the structures that were defined by a second exposure step.
The overlay is a measure for how exactly the structures produced by different masks can be arranged in relation to one another within a component. The tolerable overlay has been greatly reduced in recent years, particularly in the case of methods for multiple exposure, the use of which has become frequent in the meantime.
Overlay markers are frequently arranged on the masks in order to be able to better align the structures produced on the wafer in a plurality of exposure steps. The overlay markers can be situated outside of, or within, the structures to be imaged and usually consist of arrangements of comparatively coarse lines, the width of which typically lies in the order of 1 μm in the case of wavelengths in the DUV and VUV spectral range. Usually, the overlay markers of one exposure step are imaged exactly over the overlay markers defined in a preceding exposure step with a different mask. A measurement of the relative orientations of the overlay markers occurring after the exposure allows determination as to whether the overlay is so small that the wafer can be processed further or whether the admissible tolerances have been exceeded, which can occasionally be corrected by subsequent adjustments in the subsequent processing steps.
However, issues may arise when imaging the overlay markers onto the light-sensitive layer because the lines of which the overlay marker consists usually have a significantly greater pitch and are therefore spaced apart further from one another than the lines of the actual mask structure. For reasons of simplicity, the lines of the overlay markers are referred to as “coarser lines” on account of the greater pitch and the lines of the actual mask structure are referred to as “finer lines”, even though the width of the lines is not directly important in this context.
As already mentioned above, the mask structures can only be ideally imaged if they are illuminated by light that has an illumination angle distribution that is adapted to the mask structures. This likewise applies to the coarser lines of the overlay markers; they are often imaged in optimal fashion if they are illuminated using a conventional illumination setting in which the light is incident on the mask from all sides at small angles. Then, a central circular disk is illuminated in the pupil. By contrast very narrow parallel lines, as often occur in the mask structures, often involve a dipole setting in which the light is only incident on the mask from two opposing sides at relatively large angles of incidence.
If the coarser lines of the overlay markers are illuminated by a dipole setting, this leads to a very small depth of field when imaging the overlay markers. The reason for this lies in the fact that the occurring diffraction angles are small on account of the greater pitch. As a result, the projection light that is incident obliquely in the case of a dipole setting is hardly deflected. This is why each illumination pole produces an extremely non-telecentric image; i.e., the beams that are incident on the light-sensitive layer do not extend in an axis-parallel fashion but are strongly inclined. The depth of field is correspondingly low as very small axial displacements of the light-sensitive layer lead to a significant lateral offset of the image in that case. The consequence of this is clear rounding of the edges in the images of the overlay markers. As a result, it can become difficult to determine the location of the lines with the desired accuracy.
A further issue when imaging overlay markers consists of imaging aberrations of the projection lens having different effects on the imaging of the coarser lines of the overlay markers on the one hand and the finer lines of the actual mask structure on the other hand.
Consequently, it would be ideal if the overlay markers could be illuminated using a different illumination setting to the remaining mask structures. However, the projection light has the same illumination angle distribution at all locations in the illumination field in conventional illumination systems. However, illumination systems in which the illumination angle distribution can be set within certain limits in a field-dependent manner, i.e., depending on the location of the illumination field, have already been proposed.
Thus, US 2013/0114060 A1 has disclosed an illumination system in which an optical integrator produces a multiplicity of secondary light sources which together illuminate a field plane in which an adjustable field stop is arranged. This field plane is imaged onto the mask by a field stop lens. Here, images of the entrance facets of the optical integrator overlay in the field plane and consequently also on the mask. As a result, a very uniform illumination of the mask is obtained. In order to be able to set the illumination angle distribution in a field-dependent manner, very many small optical modulators are situated in a field plane upstream of the optical integrator, the distribution of the projection light on the entrance facets of the optical integrator being able to be changed without losses due to the small optical modulators. Since each entrance facet illuminates the mask from another direction, this renders it possible to set the illumination angle distribution on the mask in a field-dependent manner.
An even more flexible approach is facilitated by the illumination system known from WO 2015/074746 A1. In order to be able to better influence the light distribution on the entrance facets of the optical integrator, micromirrors of a digital micromirror device (DMD) are imaged onto the entrance facets in that case. As a result of this, it is possible to set practically any field dependence of the illumination angle distribution—albeit at the expense of low light losses.
What both approaches have in common is that the entrance facets of the optical integrator have to be illuminated in a highly resolved and variable manner. Since the entrance facets are very small, it is very challenging from a technological point of view to produce variable intensity distributions with the desired precision in that case.